1. Field of the Invention
The present invention relates to optoelectronic devices and more particularly to electroluminescence devices using indirect bandgap semiconductors.
2. Description of the Related Art
Optoelectronic devices can be largely divided into three categories such as devices for converting electrical energy into light energy (e.g., light emitting diodes and laser diodes), devices for detecting light signals (e.g., photodetectors) and devices for converting light energy into electrical energy (e.g., photovoltaic cells and solar cells).
Among the optoelectronic devices, devices that convert electrical energy into light energy are called electroluminescence devices and when a voltage is applied to both ends of a specific material layer (a light emitting layer) of the electroluminescence device such as a light emitting diode (LED), it emits a light corresponding to an energy bandgap (hereafter, simply called “a bandgap”) of the light emitting layer.
When one or more atoms are brought together to form a solid, electron wave functions having a potential well of each atom are overlapped. So energy states to be occupied by electrons split into energy bands according to whether filled with electrons or not.
The lower band filled with electrons is called the valence band VB, the upper band not filled with electrons is called the conduction band CB and the forbidden band called a bandgap where no electrons are allowed exists between the lower and upper bands.
Thus physical characteristics (electrical characteristics) of a solid can be explained by the energy bands. Semiconductors can be defined as materials having bandgaps smaller than those of dielectric materials (insulators) and larger than those of conductive materials (conductors).
And, when energy bands of semiconductors are expressed with an energy (E) and a wave number (k or a wave vector) as shown in the E-k diagrams (FIGS. 1 and 2), energy values of the valance and conduction bands are changed by the electron-moving directions, namely, the <100> or <111> direction in a lattice. At this time, the bandgap energy Eg is defined as the difference between the conduction band minimum and the valence band maximum.
In an E-k diagram, the conduction band minimum and the valence band maximum are dependent on semiconductor materials and have the same k value as shown in FIG. 1 or different k values as shown in FIG. 2. The former represents direct bandgap semiconductors and the latter represents indirect bandgap semiconductors.
In other words, conduction band minima and valence band maxima of direct bandgap semiconductors (e.g., GaAs, GaN, etc.) exist at the Γ-point having k=0, while in indirect bandgap semiconductors (e.g., Si, Ge, etc), valence band maxima exist at the Γ-point but conduction band minima exist at the other points (points forming an X- or an L-valleys).
FIG. 3 shows the E-k diagram of an indirect bandgap semiconductor having a local conduction-band minimum that exists at the Γ-point and an energy difference ΔEc1 from the conduction band minimum.
To obtain transitions of electrons between the bands, both energy and momentum must be conserved before and after each transition.
By the energy conservation, photons having the wavelength λ [nm] of Equation (1) corresponding to a bandgap Eg [eV] are absorbed or emitted during the transition of electrons between the bands.λ=1240/Eg  (1)
And, by the momentum conservation, momenta of photons are extremely smaller than those of transited electrons. Thus, according to Equation (2), the transited electrons have almost the same k value in the E-k diagram before and after each transition.P=h/λ=k  (2)
where P is a momentum, h is Planck's constant, λ is the wavelength,  is h/2π, and k is a wave number.
For this reason, electroluminescence devices have been mainly formed with direct bandgap semiconductors until now.
By the way, the conventional electroluminescence devices using direct bandgap semiconductors have some problems. First, the manufacturing cost is too high to grow a light emitting layer (i.e., an active layer) with direct bandgap compound semiconductors on a costly compound semiconductor substrate. Second, the conventional electroluminescence devices cannot be integrated together with common circuit elements generally fabricated on a silicon substrate.
Furthermore, because of many advantages of silicon (Si), there are many studies for using Si to form a substrate of electroluminescence devices.
However, most of the studies are to grow a light emitting layer with compound semiconductors as direct bandgap materials on one side of silicon substrate, to increase the degree of integration with circuit elements, or to more reduce a forward-biased voltage Vf in an heterojunction of Si and compound semiconductor (refer to patent reference 1 as Korean Publication No. 10-2007-0122509). The conventional technologies have problems that entail extra processes and costs to especially form a costly light emitting layer on a silicon substrate.
Furthermore, there is a try that silicon is used to a light emitting layer of an electroluminescence device, but, as mentioned above, because silicon is an indirect bandgap semiconductor, conduction band minimum and valence band maximum are placed on different k values from each other. Thus direct transitions of electrons are almost impossible.
To overcome this problem, patent reference 2 (U.S. Pat. No. 5,917,195) discloses a technology for a light emission, as shown in FIG. 4, by coupling between holes located at valence band maximum and electrons transported from an X-valley to a Γ-valley through changing the energy and momentum of electrons by lattice resonators such as phonon resonators 1 in several resonating layers made with the radioactive isotopes of silicon Si28, Si29 and Si30. In this case, difficult processes to form several resonating layers using the radioactive isotopes of silicon Si28, Si29 and Si30 are needed.
And according to non-patent reference 1 (Jifeng Liu et al., Band-Engineered Ge-on-Si Lasers, IEDM, pp. 146-149, 2010), a lattice structure of germanium Ge of an indirect bandgap semiconductor is modified by a tensile strained, as shown in FIG. 5, for changing an energy band of conduction band, in other word, for regulating to raise the X-valley having a conduction band minimum and to lower the Γ-valley at k=0 to the similar energy of the X-valley, and then some electrons injected by doping of n-type impurities enable the transport into a Γ-valley for recombining with holes located at the valence band maximum. In this case, difficult processes to especially modify a lattice structure for forming a light emitting layer with an indirect bandgap semiconductor are required.
Therefore, because technologies for efficiently using indirect bandgap semiconductors such as silicon and germanium to form a light emitting layer have not been developed, electroluminescence devices show the limitations of fabrication costs and various applications.